As soon as you start to fall behind, you have to make even greater effort to do the ADC's #1 job:
Don't die. If you're dead, you're not getting XP or gold. When you die, you put the opponents further ahead.
Farm where you can, catch up where you can. Do not risk dying, because it just makes things worse. Work with your team, try to deal damage to opponents from behind the team.
A recent game of mine I was garen, and our Lucian was way behind their ADC. He played it calmly, did not die, caught up, and ended up doing just fine.
This doesn't always work, but it's what you should try to do. (That's my take on it, anyway)
